May 5, 1881] 



NA TURE 



13 



feet in diameter) is placed a 12-inch refractor by Alvan 

 Clark, and it is intended to have in the other two an 

 equatorial for photographic work, and an altazimuth or 

 comet-seeker. It seems a pity that the use of so magni- 

 ficent a building should in some measure be sacrificed to 

 architectural display, for the splendid soxaSx faj^ade being 

 devoted to domestic purposes, an enormous proportion of 

 the observations will of necessity be made over the 

 chimneys of the dwelling-house. The great central 

 tower is 45 feet in diameter, and its dome and the 

 revolving machinery to work it have been supplied by 

 Mr. Howard Grubb, who has also put up all the domes of 

 the smaller towers. The great dome is of a very peculiar 

 construction. It is formed of two thin shells of steel plate 

 varying in thickness ; these are riveted on the inside and 

 outside of a very light set of steel plate girders 18 inches 

 deep at base and 9 inches at crown, the whole forming a 

 cellular construction like the top and bottom of the 

 Britannia Tubular Bridge. This form gives enormous 

 stiffness for the amount of material used, besides pos 

 sessing several points of peculiar usefulness for astro 

 nomical work, such, the more specially, as keeping the 

 temperature of the dome wonderfully constant, e\en 

 under most trying circumstances. The total weight of 

 this steel dome, with its ribs and'girders, the cast-iron sole 

 plate, and fitting, is about 15 tons, and as the result of a 

 series of very ingenious mechanical contrivances thought 

 out by Mr. Grubb, the tractive force necessary to pull 

 round this huge drum, even when resting, as at Dubhn, 

 on a temporary support rmd insufficiently levelled, was 

 only 70 lbs. All these domes were constructed, so far as 

 the framework went, in Dublin, and they have been placed 

 for some time past /// situ at Vienna, under the supeim 

 tendence of Mr. \V. K. Davis, Mr. Grubb's engineer. 



The new Observatoiy having been well advanced in 1874 

 Director Littrow sent his first assistant (now the Genenl 

 Director), Dr. Ed. Weiss, on a tour of inspection to 

 all the great observatories and astronomical workshops 

 of Europe and America, with the result that Dr. Weiss 

 recommended to his Government that an instrument of 

 at least 26-inch aperture should be ordered from the 

 establishment of Mr. Howard Grubb, and in the )eir 

 1875 'te contract between the Austrian Government and 

 Mr. Grubb was signed for a 27-inch refractor. The 

 mechanical parts were nearly finished in the year 1878, 

 but the greatest difficulties were experienced by the 

 Messrs. Fell of Paris in obtaining perfect disks of glass 

 for the objective, and it was not until after several triaL 

 and towards the close of 1879 that this firm succeeded m 

 sending to Dublin disks that ultimately proved perfect 

 These had to be worked into the objective at Mr. Grubb s 

 establishment, and on several occasions serious flaws 

 were only discovered at a time when but for them the 

 object-glass would have been complete. 



The general form of the equatorial may be described 

 as a modification of the German form. In designing it 

 Mr. Grubb kept in view the fact that while circumpolar 

 motion was very desirable — indeed almost necessary — 

 for objects from the horizon to, or approaching to, the 

 zenith, it was by no means so essential for objects beyond 

 that to the pole. This will be evident on consideration, 

 for nineteen-twentieths of the objects usually under 

 obsen^ation in these latitudes are between the zenith and 

 south horizon, and if one be observed north of the zenith 

 its apparent rate of progression is so slow that a very 

 little motion of the telescope takes place for any given 

 duration of observation. Keeping this in view, Mr. 

 Grubb has adopted the form shown (Fig. i), in which the 

 intersection of the axes c is placed, not over the centre of 

 the pier, but over the north end nearly, and this allows 

 the telescope circumpolar motion for all objects up to the 

 zenith. It should also be observed that this circumpolar 

 motion gives another advantage besides that of non- 

 reversal, viz. that it allows choice of two positions of the 



telescope in observing almost any object. In obsei-ving 

 near the meridian the telescope may be used cither to the 

 east or west of the pier, and in observing towards the 

 east or west the telescope can be used either over the 

 polar axis or under. This is sometimes found to be a 

 great convenience. The above considerations were those 

 which influenced j\lr. Grubb in deciding on the particular 

 form for the Vienna instrument, and the result is that 

 almost all the advantages of circumpolar motion are 

 obtained without any serious counterbalancing disad- 

 vantages. 



To enter into full details of all the various parts of this 

 magnificent telescope would far exceed the space at our 

 command. Up to this it has been thought impossible 

 to apply to large equatorials those many important con- 

 trivances to save time and labour which all first-rate 



instrument-makers would adapt to small instruments, but 

 in this one all such contrivances and many novel adapta- 

 tions of such have been utilised. To enable our readers 

 to form some idea of the resources within the ready reach 

 of the observer, we, through the kindness of the proprie- 

 tors of our contemporary, Engineering, give an iUustra- 

 tion (Fig. 2) of the eye-end of the tube. 



In the largest telescope until now in existence, the 

 great 26-inch refractor at Washington, the oft-recurring 

 operation of reading the circle involved the sending of 

 an assistant with a hand-lamp to climb up some twenty 

 feet of the instrument, and the vernier not being in a 

 fixed place, but moving about with the telescope, the 

 labour and difficulty of this operation are great. In the 

 Vienna instrument Mr. Grubb has so contrived it that 

 the observer sitting in his chair can read the right ascen- 

 sion and declination circles through one single reader, 

 all being illuminated by one gas-lamp hung on the end of 

 the declination axis. 



Another wonderful convenience is enabling an assistant 



